Facts and Glitches You Don't Know, but it's GTA IV 

0:00 Hospital OOB
0:46 Shooting car window to skip animation
1:19 NPCs stop shooting when using vending machine
1:57 Ability to slow down time with cinematic mode
2:49 No damage by jumping off bike after high jump
3:10 Cash registers always having money
4:48 Phones blocking shots
5:42 Take vehicles without activating stunt trials
6:37 Ragdoll shooting with one-handed weapons
6:55 Free-fall shooting
7:35 Police will use whatever vehicles available
8:53 How FPS impacts the darts minigame
9:16 Police are not observant
9:51 Taxi Light Detail
10:11 Emergency Vehicles Clear Traffic
11:03 NPCs phones mimic your phone
11:40 Westdyke Casino OOB
Do these glitches really work in GTA IV?
Yes, these facts and glitches work similarly the way they do in GTA V, albeit some of them having a few quirks compared to their GTA V counterpart.
Aside from a few "visual improvements" ;) and some scripts (Scripthook and Native Trainer by sjaak327), the game is pretty much vanilla so these facts and glitches should also work the same way on a vanilla copy.
Backstory
I played GTA IV quite a lot back in 2009 through 2012 so I've already encountered most of these facts and glitches before GTA V even came out.
- Nerdy stuff. Read if you're bored. -
FNG #39 (9:51) - Taxi Light Detail
Quite difficult to read due to GTA IV's texture limitations but the bottom part of the shoe ad reads "you'll never have to tie your shoes anymore."
FNG #15 (1:19) - NPCs stop shooting when using vending machine
Hostile NPCs ignore the player while button inputs are disabled, such as when drinking from a vending machine.
In GTA V, the player regains control once the entire animation finishes which is why enemies only begin shooting at the end of the animation.
In GTA IV, the controls are regained midway through the drinking animation which is why enemies shoot earlier even before the animation finishes.
FNG #24 (1:57) - Ability to slow down time with cinematic mode
Cinematic mode is enabled by holding down Capslock or by cycling through the various vehicle camera modes. Slow motion in cinematic mode can be toggled on and off using the honk button in GTA IV.
FNG #34 (5:42) - Early (vehicle)
What I think happens is that there's an invisible mission marker on where the stunt vehicle spawns. Pushing the vehicle away causes it to leave its invisible mission marker and the trials won't activate when the vehicle is entered, and placing the vehicle back to its original spot will reactivate the stunt trials if the vehicle is re-entered by the player.
It doesn't have to be the very specific vehicle you stole in order to reactivate the stunt trials. In the shown example, you may use ANY PCJ-600, park it on the spot where the mission triggers, and re-enter the bike.
This trick works on any vehicle with stunt trials in the original trilogy, and apparently, works in the definitive trilogy too.
FNG #34 (6:37) - Ragdoll shooting
Ragdolling on command isn't a thing in GTA IV so gotta use the next best way to do it.
FNG #36 (7:35) - Police will use whatever vehicles available
This only works with two or more wanted levels. With a single wanted level, cops will simply proceed to arrest the player on foot. They will not pursue the player using a vehicle with just one star.

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